This invention relates generally to multi-phase data transmission and more particularly to Time Division Multiple Access (TDMA) radio systems employing multiphase modulation in which rapid phase acquisition is important. This invention is related to U.S. applications Ser. Nos. 128,926 "Phase-Coherent TDMA Quadrature Receiver for Multipath Fading Channels" and 128,677 "TDMA Radio System Employing BPSK Synchronization for QPSK Signals Subject to Random Phase Variation and Multipath Fading", each filed on behalf of David E. Borth et al., filed on the same date as the present invention, and containing related subject matter.
In a Time Division Multiple Access (TDMA) radio system, or any communications system generally where fast acquisition and a high data rate are important considerations, a receiver is required to receive short bursts of data from one or more transmitters, each in its own timeslot. For each timeslot, a receiver using a coherent demodulator must rapidly acquire a phase reference in order to properly decode the data transmitted in that timeslot. Typically, each transmitter sends an acquisition sequence (preamble) preceding the timeslot data for this purpose. When coherent detection techniques are employed, the receiver typically regenerates the transmitted carrier phase from a carrier regeneration circuit of some type.
One common multiple-phase data modulation technique is Quadrature Phase Shift Keying (QPSK) in which half of the data to be transmitted is modulated on a carrier having 0.degree. (and 180.degree.) phase ( the I channel) and half is transmitted on a quadrature carrier (the Q channel) at 90.degree. (and 270.degree.). This signal may be transmitted over a radio channel having a random and highly variable shift in the phase. Upon reception, a reference must be established in order that the I and Q channels be identified so that the data can be properly recovered. Previous techniques in resolving the I and Q channel phases have utilized acquisition sequences that generally were different or independent in the I and Q channels. It is also known that the phase of the receiver local oscillator may be varied in order to correct for the phase offset introduced by the radio channel path. However, when high speed TDMA communications over a variable radio channel is contemplated, a more rapid acquisition method is desirable.